1. Field of the Invention
The invention relates to power supply systems. The present invention relates more precisely to a back-up power supply of the type serving to compensate temporarily for main power supply deficiency. The invention is particularly applicable to DC-to-DC converters.
2. Description of the Related Art
FIG. 1 is a diagram summarizing a prior art power supply system.
The power supply system shown in FIG. 1 is designed to power a load 10, e.g. constituted by a DC-to-DC converter delivering an output voltage Vs on the basis of a DC input voltage Ve. The voltage Ve is generated by a main power supply 15 and it can vary over a range defined by a minimum input voltage Ve1 and by a maximum input voltage Ve2. The voltage Ve is applied to the load 10 via a diode 11. Under normal operating conditions, the load 10 is thus powered by the voltage Ve.
In order to mitigate a momentary absence of voltage Ve, e.g. resulting from a micro-interruption in power supply, a back-up power supply is provided constituted by a capacitor 13 pre-charged to the voltage Ve (ignoring the voltage drop through the diode 11) via a current-limiting resistor 12. When the voltage Ve becomes lower than the voltage stored in the capacitor 13, a diode 14 whose cathode is connected to the cathode of the diode 11 becomes conductive. The resistance of the diode 14 in the conductive state then becomes much lower than the resistance of the resistor 12, and the capacitor 13 discharges into the load 10 via the diode 14. The diode 11 thus makes it possible to prevent the capacitor 13 from discharging into the cell of the main power supply 15.
The problem posed by a back-up power supply of that type is that the nominal voltage of the capacitor 13 is chosen as a function of the maximum voltage Ve2 that can be reached by the input voltage Ve. By way of example, if the voltage Ve fluctuates over the range 18 volts to 36 volts, the capacitor 13 is chosen to withstand the nominal voltage immediately above 36 volts, i.e. 50 volts. However, the load 10 can operate correctly with a power supply voltage lower than the voltage Ve2, e.g. 15 volts. This applies typically to DC-to-DC converters operating (i.e. delivering a constant regulated output voltage Vs) over a wide range of input voltages, e.g. 15 volts to 36 volts. The minimum operating voltage (15 volts) is referred to as the "threshold value", and is referenced Vmin. In the event that the main power supply fails, the capacitor 13, which is charged to a voltage Vc equal to Ve prior to the power supply interruption, discharges continuously into the load 10 until the voltage delivered by the load 10 reaches Vmin. The duration dt during which the capacitor discharges from Vc to Vmin (ignoring the voltage drop through the diode 14) is the duration for which an absence of the voltage Ve is transparent for the load 10.
Unfortunately, the duration dt is not controllable, and in certain applications, it is longer than necessary. By way of example, when specifications specify that an absence of the main power supply Ve must be tolerated for 30 ms (typically a micro-interruption in power supply), it is not necessary to provide a back-up power supply making it possible for the load 10 to operate for any longer than specified because that involves providing an energy-storage capacitor 13 that is of larger volume, and costs more.
Another known solution consists in using a low-capacitance capacitor 13, and in charging it to a high voltage, e.g. by means of a boost circuit, or by means of an auxiliary winding of a transformer. The product C.times.V is then larger, and it makes it possible to obtain a longer sustain time.
Unfortunately, that solution is costly, and switching over is complex when going over from the main power supply to the back-up power supply.